The present invention relates to ceramic matrix composite products known in the art as hybrid composites, comprising combinations of long reinforcing fibers and short reinforcing whiskers, chopped fibers, or particulates. More particularly, the invention relates to hybrid ceramic matrix composites of laminar structure.
It is known to vary the fiber content and/or the arrangement of the fibers in both ceramic and in plastic or polymer based fiber-reinforced composites. In the case of fiber-reinforced plastics, for example, U.S. Pat. No. 4,536,438 to Bishop discloses a composite structure wherein layers of woven fibers are alternated with layers of parallel-oriented fibers in order to improve composite strength. The patent teaches that, in polymer systems of this type, the points of fiber intersection in woven layers in the composites were identified as points of weakness, and the inclusion of directed fiber layers was found to provide composites with better residual strength after impact.
Design considerations for ceramic matrix composite materials are significantly different than for fiber-reinforced polymer systems. In ceramic composites, the matrix is a brittle material and the reinforcing fibers generally have higher elastic moduli and higher failure strain than the matrix. The art has recognized that one way to improve the off-axis (i.e., transverse) properties of these composites is to improve the strength and/or toughness of the matrix material.
It is known to incorporate inorganic whiskers or chopped fibers in ceramic matrix materials utilized for the fabrication of ceramic matrix composites in order to increase the microcrack yield point of the matrix. Thus, for example, U.S. Pat. No. 4,615,987 describes glass-ceramic matrix composites comprising a combination of reinforcing fibers and whiskers, while U.S. Pat. No. 4,626,515 discloses a similar hybrid fiber/whisker reinforcement combination for glasses.
Depending upon the ceramic matrix system and combination of fibers employed, these additions have been found to significantly improve the transverse strength of the composite, which is the strength of the material in directions of flexure transverse to the predominant direction of the fiber reinforcement in the material. In addition, improvements in the interlaminar shear strength of these composites, which is the stress at which delamination of adjoining fiber-reinforced layers in the composite have been observed.
There are, however, disadvantages in adding whiskers and/or short fibers to these composite systems to improve matrix performance. For example, the introduction of whiskers into the matrix produces a small but definite reduction in the ultimate strong-axis failure strain exhibited by the resulting hybrid composites. This effect is presently attributed primarily to fiber weakening caused by surface damage to the fibers inflicted by the added whiskers.
Another disadvantage associated with whisker additions to fiber composites appears to be a reduced resistance to high temperature oxidation damage shown by the fibers. The effect in this case is attributed to damage caused by the added whiskers to protective coatings or other protective layers present on the fiber surfaces.
It is a principal object of the present invention to provide novel structures for hybrid ceramic matrix composites wherein high interlaminar shear strength and high transverse strength may be achieved without compromising the ultimate failure strain of the composites.
It is a further object of the invention to provide a method for improving the off-axis properties of fiber-reinforced ceramic matrix composite materials without adversely affecting the strength properties of reinforcing fibers disposed therein.
Other objects and advantages of the invention will become apparent from the following description thereof.